A common form of underwater breathing apparatus is the open circuit type, an example of which is illustrated in FIG. 1. The user inhales from a cylinder 2 of compressed air (or other breathable gas) via an automatic demand valve 4 having a mouthpiece 6. The demand valve includes a flexible diaphragm 8 exposed to ambient pressure on one side and the mouthpiece on the other side, such that the pressure reduction at the mouthpiece caused by inhalation by the user deflects the diaphragm towards the mouthpiece. This urges the diaphragm against a lever 10, deflection of which opens a valve 12, thereby allowing air to flow from the cylinder 2 to the user. The user simply exhales to the environment via an exhaust valve 14.
Although simple and robust, an open circuit system of the type shown in FIG. 1 has numerous disadvantages, including:                short and uncertain endurance, reduced further by increases in depth and/or breathing rate;        massive wastage of breathing gas, requiring user to carry a large and heavy cylinder (80% of air is unwanted nitrogen and only a small proportion of the oxygen content inhaled is actually used);        nitrogen is absorbed into the blood at depth, leading to narcosis and a risk of decompression sickness;        air from the tank is dry and cold, dehydrating and chilling the diver.        
An alternative to the open circuit type of system shown in FIG. 1 is a closed circuit rebreather, in which the exhaled gas is scrubbed of carbon dioxide, captured in a bag, replenished with oxygen and returned to the user. An early example of such a system is shown in FIG. 2. The system defines a breathing loop and includes one-way valves 20 and 22 at the mouthpiece 24 which only allow gas to flow one way around the loop. Exhaled gas passes through a carbon dioxide scrubber 26 into a breathing bag or counterlung 28. When the user inhales, this reduces the pressure in the loop, causing automatic demand valve 30 to open, allowing gas to flow from a compressed oxygen cylinder 32 into the counterlung 28.
In comparison to the open circuit system of FIG. 1, the closed circuit arrangement of FIG. 2 is relatively compact and light, as an endurance of several hours is possible regardless of breathing rate using a relatively small oxygen cylinder. The gas in the loop is warmed by the user and there is a stealthy lack of bubbles.
A problem with the system of FIG. 2 is that, beyond a certain ambient pressure, oxygen itself becomes toxic to the body, giving rise to symptoms similar to an epileptic fit. Different people have different susceptibility to this, and so the use of pure oxygen is only safe at depths of less than six meters. To safely go deeper, it is necessary to dilute the oxygen with some other gas such as air.
More recent developments in this field led to a fully closed circuit mixed gas rebreather, as exemplified by the system of FIG. 3. A supply of oxygen to the breathing loop is maintained via a control device 34. This control may be provided electronically, for example by placing oxygen sensors such as fuel cells in the loop. Should their output voltage drop below a preset level, an electric valve in control device 34 opens to inject a burst of oxygen. Alternatively, control device can simply provide a steady feed of oxygen, of the order of one liter per minute. In that case, the control device may be in the form of a small orifice, made from ruby for example. The oxygen in the breathing loop is diluted by gas from a cylinder 36 of a suitable compressed diluent gas. The diluent gases typically used in underwater breathing systems are air or an oxygen/helium mix, for example. This gas is fed to the loop via automatic demand valve 30.
As the user swims deeper, and the gas in the loop is compressed by the surrounding water pressure, the volume of counterlung 28 is topped up by the diluent gas, allowing the diver to take a full breath. Thus, the user is given a high percentage of oxygen at the water's surface, becoming more dilute with depth.
However, the safety record of systems of the form shown in FIG. 3 is poor, the main cause of these accidents being hypoxia (that is, insufficient oxygen) as a result of the system becoming incapable of supplying the diver with sufficient oxygen. This can occur through a failure in the system (such as a blocked orifice, empty tank or flat battery), user error (for example accidentally turning off the oxygen supply), or challenging circumstances such as high levels of exertion, rapid ascent from depth (though the percentage of oxygen stays the same, the concentration drops as the gas expands), panic (heavy breathing and exhalation through the nose) or a combination of these factors. When oxygen is used up faster than it can be replaced, the breathing loop volume drops, as the carbon dioxide produced is removed by the scrubber, the user cannot inhale fully and the automatic demand valve is actuated, replacing the “missing” oxygen with air. As what little oxygen in this air is used up too, the cycle repeats itself, and the mixture rapidly becomes incapable of supporting life. Furthermore, without the presence of carbon dioxide (the stimulus for feeling out of breath), the diver is unaware of there being a problem.